As the number of mobile data subscribers and bandwidth-intensive services competing for wireless resources has continued to grow, wireless network operators have responded to increased demands by increasing capacity through addition of small cells that are tightly integrated with existing macro networks to spread traffic loads. The overall result of this is to maintain performance and service quality while reusing spectrum efficiently.
Small cells may be introduced into macro networks through the addition of lower power base stations, such as eNBs and HeNBs, to existing macro eNBs. Site acquisition for such smaller, low power base stations is easier and less expensive than the addition of macro eNBs. In generally, small cells are deployed to increase capacity in areas of high user demand (“hot spots”), as well as to fill in areas not already covered by the macro network. Small cells also improve overall network performance and service quality by offloading traffic from the macro cells. The result is a heterogeneous network, or “HetNet,” in which larger macro cells are deployed in combination with small cells.
In HetNets, the various size cells may be referred to as macro cells, micro cells, pico cells, and femto cells (in decreasing order of base station power). Cells other than macro cells (e.g., micro cells, pico cells, and femto cells) may be referred to collectively as “small cells.” The actual size of each cell type depends on the power of the corresponding eNB, as well as the eNB's antenna position and location environment (e.g., indoor vs. outdoor; rural vs. urban). An HeNB is a low power eNB used primarily for providing indoor coverage femto-cells for Closed Subscriber Groups (CSG). HeNBs, are typically privately owned and deployed without coordination with the macro-network. If the frequency used in the femto-cell is the same as the frequency used in the macro-cells, and the femto-cell is only used for CSG, then there is a risk of interference between the femto-cell and the surrounding network.